1. Field of the Invention
The invention relates to a process for the continuous preparation of addition-crosslinking liquid silicone rubbers.
2. Background Art
EP 0 568 891 A1 (DE 4 215 205) describes a process for the continuous preparation of liquid silicone rubbers. The object was to provide a process for the complete continuous compounding of crosslinkable silicone rubbers, in particular for the parallel preparation of two components of liquid silicone rubbers, the stated polymers also being relevant to the HTV product sector. The publication claims a multistage process with a hydrophobicizing reaction in the first three stages (e.g. using HMN), followed by division of the material into an A stream and B stream, with admixture of small amounts of the necessary additives by way of (coolable) static mixers.
There are operational difficulties with the foregoing process, due to difficulty of dealing with safety aspects, such as the removal of oxygen from the silica feed, which may pose an explosion risk, the conduct of the reaction with return of the treatment agent, and also the degassing and temperature control which is important for admixture of temperature-sensitive additives in the A/B streams. The result is a very long residence time, and therefore dead time within which the process cannot be controlled. The efficiency of incorporation of additives into the A component and B component by way of coolable static mixers is variable, in respect of both temperature reduction and especially mixing quality. The use of static mixers to introduce very small amounts which decisively affect the material is a process which either becomes very complicated or is unreliable.
DE 198 09 548 A1 describes a process for the continuous preparation of moisture-crosslinkable organopolysiloxane compositions. An object was to provide a process for the continuous preparation of RTV-1 compositions while minimizing the losses of time and RTV-1 composition resulting from a mixing specification change (e.g. color). That publication claims a three-stage process in which a crude mixture is first prepared from α,ω-dihydroxypolydiorganosiloxanes and a hydrophilic filler, and after degassing, this mixture is treated with additives, such as crosslinkers and condensation catalysts, by means of a dynamic mixer. Because this mixer has small volume, its cleaning produces only a small loss of time and material on product change. The RTV-1 compositions contain relatively few starting materials, weight variations of which have a less severe effect than is the case with addition-crosslinking systems. Emphasis is placed on minimization of losses, specifically on product change.